In general, a ventricular assist device is used in case where a medical treatment is no longer effective to a heart failure patient or in case where it is difficult to treat a heart failure by open heart surgery. The ventricular assist device is employed to replace a ventricle function until heart transplantation become available or to reduce a heart load in a restoration period.
One thirds of the patients having a ventricular assist device go through heart transplantation and one fifths thereof are restored with no heart transplantation. Ventricular assist devices are assorted into an implant-type ventricular assist device and an extracorporeal ventricular assist device depending on the grafted regions. Furthermore, the ventricular assist devices are classified into a left ventricle assist device, a right ventricle assist device and a left-right ventricle assist device depending on the heart portions assisted by the ventricular assist device. The left ventricle assist device is widely used. Moreover, the ventricular assist devices are assorted into a pneumatic type and an electric type depending on the kinds of energy used. In addition, the ventricular assist devices are classified into a pulsating type, a centrifugal type and an axial flow type depending on the kinds of actuators (pumps) that ejects blood. An implant-type left-right ventricle assist device may be regarded as one kind of artificial hearts but differs from a perfectly-transplanted artificial heart through which blood is circulated with a natural heart removed.
FIG. 1 shows a representative intracorporeal-type ventricular assist device (HeartMate, a product of Thoratec Corp., U.S.A.). Referring to FIG. 1, the ventricular assist device includes an inlet conduit 4 connected to the top portion 3 of the left ventricle, an outlet conduit 6 connected to the aorta 5 and a blood pump 7 connected to the inlet conduit 4 and the outlet conduit 6. The blood pump 7 includes a blood bag (not shown) connected to the inlet conduit 4 and the outlet conduit 6, a squeezer (not shown) for squeezing the blood bag so that the blood introduced into the blood bag through the inlet conduit 4 can be ejected through the outlet conduit 6, and an electric motor (not shown) connected to the squeezer. One-way valves permitting blood to flow only in one direction are installed in the inlet conduit 4 and the outlet conduit 6. A battery and a control unit 9 are provided outside the body and are connected to the blood pump 7 through electric wires 8 penetrating the skin. The ejection of blood is controlled by the control unit 9. The battery serves to supply electric power to the control unit 9 and the electric motor.
The ventricular assist device cited above is provided with only one blood bag. Therefore, if the blood stored in the blood bag is ejected once, the blood can be ejected again only after the blood is sufficiently gathered in the blood bag. In other words, if the blood bag is squeezed, the volume of the blood bag is reduced whereby the blood flows out from the blood bag but does not flow into the blood bag. To the contrary, if the blood bag is released, the volume of the blood bag is increased due to the elastic restoration of the blood bag whereby the blood flows into the blood bag but does not flow out from the blood bag. In this manner, the inflow and outflow of the blood occurs alternately over time in the conventional ventricular assist device. This poses a problem in that the inflow and outflow of the blood is performed in a quite inefficient manner.
In case of the implant-type ventricular assist device, the volume of the blood bag undergoes a change before and after the blood bag is squeezed. In order to compensate the changed volume, it is necessary to employ a volume compensator or a vent line extending to the outside of the body. In other words, a volume compensator or a vent line for re-inflating the blood bag is needed in order to allow the blood to flow into the blood bag after ejection thereof. Use of the volume compensator or the vent line tends to make the ventricular assist device complex. In particular, the vent line may become a cause of increasing the risk of infection.